Tetrasilicic mica glass-ceramic article

ABSTRACT

This invention relates to tetrasilicic fluorine mica glassceramic compositions consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 45-70 percent SiO2, 8-20% MgO, 8-15% MgF2, a total of 5-35% (R2O + RO), wherein R20 ranges from about 5-25% and consists of one or more oxides selected in the indicated proportions from the group consisting of 0-20% K2O, 0-23% Rb2O, and 0-25% Cs2O, and wherein RO ranges from 0-20% and consists of one or more oxides selected from the group consisting of SrO, BaO, and CdO, a total of 0-10% of oxides selected from the group consisting of As2O5 and Sb2O5, and up to about 5% of glass colorants. Tetrasilicic mica glassceramic articles produced from these compositions exhibit good machinability with steel tools, good mechanical strength, moderate thermal expansion, and good chemical durability. The use of the optional constituents As2O5, Sb2O5, and the glass colorants permits the production of translucent glass-ceramics and glass-ceramics having the appearance of marble.

United States Patent 1 Grossman Oct. 1, 1974 [75] Inventor: David G. Grossman, Painted Post,

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: Mar. 14, 1973 [21] Appl. No.2 341,038

Related US. Application Data [63] Continuation-impart of Ser. No. 117,933, Feb. 24,

1971, Pat. NO. 3,732,087.

[52] US. Cl 106/39.6, l06/39.8, l 06/52, l06/73.l, l06/D1G. 3 [51] Int. Cl. C03c 3/22 [58] Field of Search l06/39.6, 33, DIG. 3; 65/33 [56] References Cited UNITED STATES PATENTS 3,022,179 2/1962 Morrissey 106/39.6 3,756,838 9/1973 Beall 106/52 FOREIGN PATENTS OR APPLICATIONS 848,447 1956 Great Britain roe/39.6 1,268,125 6/1961 France 106/39.6 OTHER PUBLICATIONS Berezhnoi, A.l.Glass-Ceramics and Photo$italls,

(1970). Plenum Press, N.Y., pp. 245-248. (1970);

Primary Examiner-Helen M. McCarthy Attorney, Agent, or Firml(ees van der Steere; Clarence R. Patty, Jr.

[5 7 ABSTRACT This invention relates to tetrasilicic fluorine mica glass-ceramic compositions consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 45-70 percent SiO 8-20% MgO, 8l5% MgF a total of 535% [R 0 R0], wherein R 0 ranges from about 5-25% and consists of one or more oxides selected in the indicated proportions from the group consisting of 0-20% K 0, 023% Rb O, and 025% C5 0, and wherein R0 ranges from 020% and consists of one or more oxides selected from the group consisting of SrO, B210, and CdO, a total of 0-10% of oxides selected from the group consisting of As O and Sb O and up to about 5% of glass colorants. Tetrasilicic mica glass-ceramic articles produced from these compositions exhibit good machinability with steel tools, good mechanical strength, moderate thermal expansion, and good chemical durability. The use of the optional constituents AS205, Sb O and the glass colorants permits the production of translucent glass-ceramics and glass-ceramics having the appearance of marble.

3 Claims, No Drawings TETRASILICIC MICA GLASS-CERAMIC ARTICLE CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 117,933, filed Feb. 24, 1971, now US. Pat. No. 3,732,087.

BACKGROUND OF THE INVENTION A glass-ceramic article results from the controlled crystallization in situ of a glass article. Hence, the manufacture of glass-ceramics normally involves three gen eral steps: first, the compounding of a glass-forming batch containing a nucleating or crystallizationpromoting agent; second, the melting of the batch to form a homogeneous liquid and the simultaneous cooling and shaping of the melt to form a glass article of the desired dimensions and configuration; and, finally, the heat treatment of the glass article so produced in accordance with a specifically defined time-temperature schedule to develop nuclei in the glass which act as sites for the growth of crystals as the heat treatment proceeds.

Since the crystallization in situ is brought about through an essentially simultaneous crystal growth on countless nuclei, the structure of a glass-ceramic article comprises relatively uniformly-sized crystals homogeneously dispersed in a residual glassy matrix, these crystals constituting the predominant proportion of the article. Thus, glass-ceramic articles are frequently described as being at least 50% crystalline and, in numerous instances, are actually over 75% crystalline by volume. In view of this very high crystallinity, the chemical and physical properties of glass-ceramic articles are normally materially different from those of the original glass and are more closely related to those demonstrated by the crystal phase. Also, the residual glassy matrix will have a far different composition from that of the parent glass since the components making up the crystal phase will have been precipitated therefrom.

Because a glass-ceramic article is the result of the crystallization in situ of a glass article, conventional glass forming methods such as blowing, casting, drawing, pressing, rolling, spinning, etc. can usually be employed in securing the desired configuration to an article. Also, like glass, a glass-ceramic article is nonporous and free of voids.

US. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, provides an extensive study of the practical aspects and theoretical considerations that must be understood in the manufacture of such articles, as well as a discussion of the crystallization mechanism. Reference is made thereto for further explanation of these matters.

The micas comprise a family of silicate minerals that have a unique two-dimensional layered or sheet structure. Naturally-occurring mica consisting of large, book-like crystals can readily be split into thicknesses of 0.001 inch or less. The property of flexibility, coupled with high dielectric strength, has made sheet mica a very important electrical insulating material.

Most naturally-occurring micas are hydroxyl silicates whereas many synthetic micas have been produced by replacing the hydroxyl groups within the mica structure with fluorine. Much research has been undertaken in the field of synthetic mica, and this work can be categorized into five general areas: efforts to produce single radius, Y

crystals of fluorine mica, hot-pressed fluormica ceramics, glassbonded fluormica ceramics, fusion cast mica materials, and, recently, fluormica glass-ceramics.

The crystalline structure of fluormica has been studied extensively, with the generalized structural formula being written as X,, Y Z O F where X represents cations which are relatively large in size, i.e., l.Ol .6A represents somewhat smaller cations, 0.6-0.9A radius, and Z represents small cations, 0.3-0.5A radius, which coordinate to four oxygens. The X cations are in dodecahedral coordination and the Y cations in octahedral coordination. The basic unit of the mica structure is the Z 0, hexagonal sheet, formed because each 20 tetrahedron shares three of its corners with three other such tetrahedrons in a plane. In the fluormicas, as for all micas, two Z 0 sheets, each with their apical oxygens and associated interstitial fluoride ions directed toward each other, are bonded together by the Y cations. These cations coordinate octahedrally with two apical oxygens and one fluoride ion from each Z 0 sheet. The resultant mica layer has been called a 2 to 1 layer because it consists of one octahedral sheet sandwiched between two tetrahedral sheets. The fluoride ions and apical oxygens of the tetrahedral sheets offset cations of the octahedral sheet. The mica layers themselves are bonded to each other by the relatively large X cations in the so-called interlayer sites. These X cations are normally potassium but are sometimes other large alkali metal and alkaline earth metal cations such as Na Rb Cs Ca, Sr, Cd, and Ba.

US. Pat. No. 3,689,293 to G. H. Beall describes glass-ceramic articles comprising tetrasilicic mica crystals of fluorophlogopite or boron fluorophlogopite which display a variety of interesting and useful properties.

SUMMARY OF THE INVENTION I have now discovered that glass-ceramic articles consisting essentially of tetrasilicic fluormica crystals dispersed in a minor glassy phase can be produced from relatively stable clear to opal glasses over a particularly defined composition area. These glasses consist essentially, in weight percent on the oxide basis as calculated from the batch, of 45-70% SiO 8-20% MgO, 8-l5% MgF and a total of 535% [R 0 RO], wherein R 0 ranges from about 525% and consists of one or more oxides selected in the indicated proportions from the group consisting of 0-20% K 0, 0-23% Rb- O, and 025% C5 0, and wherein R0 ranges from 020% and consists of one or more oxides selected from the group consisting of SrO, BaO, and CdO. Optional constituents which may be added to alter the properties of the glass-ceramic products include a total of 0-l0% of oxides selected from the group consisting of As O and Sb 0 and a total of 0-5% of glass colorants.

In the fluorine micas which crystallize from the glasses of this invention, the X, Y, and 2 positions are believed to be filled in the following manner: X positionK, Rb, Cs, Sr, Ba, or Cd as available; Y position-Mg only; and Z position-Si only. These micas, which normally have the postulated formula KMg Si O F are described as tetrasilicic because they do not display Alor B-for-Si substitutions in the Z 0 hexagonal sheets of the mica layer as do the fluorophlogopites (KMg Alsi O F- and boron fluorophlogopites (KMg BSi O F Thus, although the basic mica structure of the glass-ceramics of the invention, as identified by X-ray diffraction, is of the phlogopite type, having a diffraction pattern closely matching that of boron fluorophlogopite, the tetrahedral sheets are made up exclusively of SiO tetrahedra, there normally being no other cations present in the glass composition small enough to take up the four-coordinated Z positions. From a study of the X-ray diffraction data it appears that tetrasilicic fluormica is the primary crystalline species present in the finished glass-ceramics, the usual variations being in which of the K", Rb, Cs", Sr, Ba or Cd ions (or combinations thereof) occupies the X or interlayer positions. The Y positions are believed almost exclusively occupied by Mg ions, and very few deficiencies in either the X or Y positions are expected. Thus, the glass-ceramics of the present invention are related to prior art mica glass-ceramics in that they contain synthetic mica crystals, but they are distinguishable on the basis that they do not contain trivalent cations such as Al and B as essential crystal constituents.

Minor additions of other oxides to the base glass composition, such as P TiO ZrO FeO, ZnO, 6e0 MnO, La O and SnO can be tolerated to a total of about 10% by weight and may be useful, for example, in controlling the properties of the parent glass and the residual glassy phase.

In general terms, then, my invention comprises melting a batch for a glass consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 45-70% SiO 820% MgO, 8-l5% MgF a total of 535% [R 0 RO] wherein R 0 ranges from about 525% and consists of one or more oxides selected in the indicated proportions from the group consisting of 020% K 0, 0-23% Rb O, and 025% C5 0, and wherein R0 ranges from 020% and consists of one or more oxides selected from the group consisting of SrO, BaO, and CdO, a total of 0-l0% of oxides selected from the group consisting of AS205 and Sb O and up to about 5% of glass colorants, simultaneously cooling the melt at least below the transformation range thereof and shaping a glass article therefrom, and thereafter heat treating this glass article at temperatures between about 650l,200C. for a sufficient length of time to obtain the desired crystallization in situ. The transformation range has been defined as that range of temperatures over which a liquid melt is deemed to have been transformed into an amorphous solid, commonly being considered as being between the strain point and the annealing point of the glass.

The glass batch selected for treatment may comprise essentially any constituents, whether oxides or other compounds, which upon melting to form a glass will produce a composition within the aforementioned range. For purposes of convenience, the presence of fluorine in these compositions is expressed as a quantity of MgF however, MgF may or may not comprise a batch material and fluorine may be incorporated into the batch using any of the well-known fluoride compounds employed for the purpose in the prior art which are compatible with the compositions herein described.

Heat treatments which are suitable for transforming the glasses of the invention into predominantly crystalline mica glass-ceramics generally comprise the initial step of heating the glass article to a temperature within the nucleating range of about 650850C. and maintaining it in that range for a time sufficient to form numerous crystal nuclei throughout the glass. This usually requires between about A and 10 hours. Subsequently, the article is heated to a temperature in the crystallization range of from about 800l,200C. and maintained in that range for a time sufficient to obtain the desired degree of crystallization, this time usually ranging from about 1 to hours. Inasmuch as nucleation and crystallization in situ are processes which are both time and temperature dependent, it will readily be understood that at temperatures approaching the hotter extreme of the crystallization and nucleation ranges, brief dwell periods only will be necessitated, whereas at temperatures in the cooler extremes of these ranges, long dwell periods will be required to obtain maximum nucleation and/or crystallation.

My preferred heat treating practice comprises the steps of heating the glass article to a nucleation temperature between about 750850C., maintaining it in that range for about 1-6 hours, subsequently heating it to a crystallization temperature between about l,OO0l,l50C., and maintaining it in that range for about 1-8 hours.

It will be appreciated that numerous modifications in the crystallization process are possible. For example, when the original batch melt is quenched below the transformation range thereof and shaped into a glass article, this article may subsequently be cooled to room temperature to permit visual inspection of the glass prior to initiating heat treatment. It may also be annealed at temperatures between about 550-650C. if desired. However, where speed in production and fuel economies are sought, the batch melt can simply be cooled to a glass article at some temperature just below the transformation range and the crystallization treatment begun immediately thereafter.

Further, whereas a two-step heat treatment schedule is to be preferred, a very satisfactory crystallized body can be achieved when the original glass article is merely heated from some temperature below the transformation range to temperatures within the range from about 750l,l50C. and maintained within that range for a sufficient length of time to induce the desired crystallization. Also, it is apparent that no single hold temperature within any of the cited ranges is absolutely required to secure satisfactory crystallization. Rather, the temperatures may vary at will within the designated ranges. Thus, if the rate of heating the glass body above the transformation range is relatively low and the final crystallization temperature employed is relatively high, no hold period at any one temperature will be required. In cases where it is desirable to minimize deformation, I prefer to employ heating rates of about 35C./minute. These heating rates have produced little, if any, deformation throughout the composition field operable in this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As previously disclosed, all of the micas of the invention may be classified generally as tetrasilicic micas. It is presently believed that magnesium fluoride (sellaite) is the first phase to crystallize from the glass upon heat treatment, and that subsequent heating to higher temperatures rapidly causes mica crystallization at sites occupied by sellaite nuclei. However, this hypothesis is difficult to confirm since no sellaite peak has been observed in the diffraction pattern of these materials after the onset of mica crystallization. It is believed that this isdue to the disappearance of the sellaite phase during mica formation at high temperatures. In any event, tetrasilicic fluormica is the principal crystalline phase identified by X-ray diffraction which has been found to be present in quantity in the glass-ceramics of the invention. Other crystalline phases, such as enstatite and cristobalitc, have been observed only in minor amounts, if at all.

TABLE I K 0 MgF MgO SiO AS205 NiO CuO C130 Fe O Au Sn Cullet (Composition No. 9)

llll

Ill

llll lgl (in excess) Notwithstanding the fact that the crystal composition of the micas of the invention is relatively invariant, certain differences in properties may be attributed to variations in crystalline microstructure. Of course, glassceramics of lower crystallinity are expected to be more glass-like, and thus harder and less machinable, than those of comparatively high crystallinity. In addition, however, strength and machinability have been found to be dependent on the size and shape of the mica crystals produced. Generally, fine-grained glass-ceramics having numerous small crystals and/or crystals with a low aspect ratio have been found to be stronger and harder than those containing fewer large crystals of high aspect ratio, but also less machinable, whereas those containing a high percent of mica crystallinity with a large aspect ratio tend to be softer and more maqbin 'b e.

Table I records compositions, expressed in weight percent on the oxide basis as calculated from the batch, of thermally crystallizable glasses which, when thermally treated according to the method of the invention, were crystallized in situ to relatively uniformly crystalline tetrasilicic mica glass-ceramics. The ingredients making up the glass batches may be any materials,

Following annealing, the glass patties were placed in an electricallyfired furnace and subjected to the heat treatment schedules reported in Table II. Upon completion of the heat treatment, the electric current to the furnace was cut off and the crystallized articles either removed directly from the furnace into the ambient atmosphere or simply left in the furnace and permitted to cool to room temperature at the furnace rate, this rate averaging between about 35C./minute. In each schedule, the temperature was raised at a rate of about 5C./minute to the holding temperatures. Table II also records a visual description and qualitative measure of machinability of each crystallized article. Measurements of acid durability and coefficient of thermal expansion are reported where determined on individual products.

The determinations as to the machinability of the products shown in Table II were qualitative, being made on a comparative basis after drilling with steel drills and hack-sawing with conventional blades. Translucence was determined for each material by visual examination in thin cross-section under ordinary lighting conditions, again on a comparative and qualitative basis. Thermal expansion is expressed in cm/cm/C. X 10 with the value given being the average value observed over the range from O300C. The acid durability values are expressed in terms of the weight loss per unit area of a standard sample after immersion in HCl at 95C. for 24 hours, in milligrams per square centimeter. Visual examination also revealed a very high degree of crystallization in these samples; at least 50% volume crystallinity and usually significantly greater proportions up to about 90% crystallinity are indicated.

Translucence can also be improved by Controlling the composition of the base glass so that large amounts of alkali metal oxides and little or no alkaline earth metal oxides are present. However, when comparatively large quantities of alkali metal oxides are to be employed, I prefer to use K 0 rather than RlyO or Cs O, since these latter constituents do not provide any significant advantages over K 0 and add greatly to the TABLE ll Example Thermal Expansion Acid Durability No. Heat Treatment Visual Description Machinability (cm/cm/C. X (mg/emf) 1 800C. for 4 hrs. fine grain fracture, off- Fair 53.3 5.1

1090C, for 4 hrs. white, slightly translucent 2 800C. for 4 hrs. fine cherty fracture, white, Fair ll00C. for 4 hrs. slightly translucent 3 800C. for 4 hrs. fine cherty fracture, white, Fair 50.0 3.l

1 100C. for 4 hrs. opaque 4 800C. for 4 hrs. fine grain fracture, white, Fair 1 150C. for 4 hrs. opaque 5 800C. for 4 hrs. coarse sugary fracture, Excellent 48.8 12.0

lll5C. for 4 hrs. white, slightly translucent 6 800C. for 4 hrs. medium grain sandy Good 49.6 13.0

1 125C. for 4 hrs. fracture, white opaque 7 800C. for 4 hrs. medium grain cherty fracture, Excellent 12.0

ll C. for 4 hrs. white, slightly translucent 8 800C. for 4 hrs. fine grain cherty fracture, Excellent 47.9 9.2

l l25C. for 4 hrs. grey opaque 9 800C. for 4 hrs. fine grain fracture, white, Very Good 58.3 5.0

ll00C. for 4 hrs. good translucence l0 800C. for 4 hrs. very fine grain fracture, Good 74.6 9.1

1065C. for 4 hrs. white, excellent translucency ll 800C. for 4 hrs. fine grain fracture, white, Fair 1090C. for 4 hrs. good translucence l2 800C. for 4 hrs. fine grain fracture, green- Very Good 1 100C. for 4 hrs. grey serpentine marble appearance l3 800C. for 4 hrs. fine grain fracture, pale Very Good 1 100C. for 4 hrs. purple marble appearance l4 800C. for 4 hrs. fine grain fracture. pale Very Good 1 100C, for 4 hrs. tan marble appearance 15 800C. for 4 hrs. fine grain fracture, tan Very Good 1 100C. for 4 hrs. marble with yellow streaks 16 800C. for 4 hrs. fine grain fracture, Very Good l l00C. for 4 hrs. rusty red color 17 800C. for 4 hrs. fine grain fracture, dark Very Good 1090C. for 4 hrs. green marble with white streaks Expected to have physical properties similar to composition Example No. 9.

Certain composition variations in the base glass have been found to predictably alter the properties of the mica glass-ceramic products. Thus, increasing the quantities of the alkali metal oxides present in the base glass generally decreases the machinability and refractoriness of the resultant glass-ceramics, whereas increasing the presence of the alkaline earth metal oxides usually results in a more refractory product.

The most significant product variations, however, are obtained through the use of certain additions to the base glass batch or to the glass melt. Thus, translucent micas may be produced through the addition of oxides selected from the group consisting of As O and Sb O,-, to the glass batch. Translucent micas are considered for use in high temperature lamps and in decorative or ornamental applications. Generally, additions totalling between about 0.58% by weight of the batch are preferred, and additions in excess of about 10% do not appear to have any further beneficial effect on the product. While both of these additives will increase the translucence of these mica glass-ceramics, A5 0,, is preferred because it also greatly increases the machinability thereof.

cost of the batch. Glass compositions consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 55-65% SiO 12-20% MgO, 9*] 3% MgF 7-l8% K 0, and 0.58% AS205 are preferred in the manufacture of mica glass-ceramics combining good machinability with optimum translucence.

The addition of glass colorants to the translucent mica glass-ceramic compositions of the present invention has produced machinable materials capable of taking a polish and having the appearance of marble. Preferably, these colorants are added to the glass melt just prior to forming in such a way as to produce a streaking effect. Generally, any of the conventional glass color-' not deemed particularly desirable. Normally, such colorant additions will not exceed about 2% Glass compositions which are preferred in producing artificial marbles according to the present invention are those consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 5565% SiO 1220% MgO, 9l3% MgF 7-l8% K 0, 0.5-8% AS205 and from a trace to about 2% of glass colorants selected from the group consisting of K Cr O KMnO NiO, CuO, Cr O Fe O FeO, V CoO, elemental Au, and Nd203.

The appearance of artificial marbles produced according to the present invention may vary considerably depending upon the particular technique employed in utilizing the glass colorants. Normally, the colorants will be added to the base glass melt shortly prior to pouring or forming, either in the form of the colorant compound, a colored glass melt or colored glass cullet. Alternatively, the principal melt may be of a colored glass, and a white streaking effect obtained through the addition of a colorless glass melt or cullet. Numerous other variations in the above techniques will, of course, be apparent to those skilled in the art. Thus, artificial marbles may be produced in large sheets, or large slabs and blocks may be produced simply by stacking sheets and sagging them together to produce monolithic bodres.

These materials have good machinability and moderate thermal expansion, and their modulus of rupture ranges from about five to 10 times that of naturallyoccurring marble. Furthermore, their acid durability is good; 24 hour weight losses in 5% HCl at 95C. range from a high of about l2 milligrams down to about 3 milligrams or less per square centimeter of surface area. Such durability compares very favorably with that of naturally-occurring marble, which can exhibit weight losses under comparable conditions of up to about 265 milligrams per square centimeter. Such properties and characteristics make artificial marbles produced according to the present invention eminently suitable for building, ornamental and statuary purposes and the like.

I claim:

1. A glass-ceramic article consisting essentially of tetrasilicic fluormica crystals dispersed in a glassy matrix, said crystals constituting at least 50% by volume of the article and being essentially free of Al and B as crystal constituents, and said article being formed by the crystallization in situ of a glass body consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 55-65% SiO 12-20% MgO, 9l3% MgF 535% (R 0 R0) wherein R 0 ranges from 525% and consists of at least one oxide selected in the indicated proportion from the group consisting of O-20% K 0, O23% Rb O and 0-25% C5 0, and wherein R0 ranges from 020% and consists of at least one oxide selected from the group consisting of SrO, BaO and CdO, O-l0% total of oxides selected from the group consisting of As O and Sb O and 02% total of glass colorants selected from the group consisting of K Cr O KMnO NiO, CuO, Cr O Fe O FeO, V 0 CoO, Ndgog and elemental Au.

2. A glass-ceramic article consisting essentially of tetrasilicic fluormica crystals dispersed in a glassy matrix, said crystals constituting at least 50% by volume of the article and being essentially free of Al and B as crystal constituents, and said article being formed by crystallization in situ of a glass body consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 5565% SiO l2-20% MgO, 9l3% MgF 7l8% K 0 and O.58% AS205.

3. A glass-ceramic article according to claim 2 which additionally contains from a trace to about 2% of glass colorants selected from the group consisting of K Cr O KMnO NiO, CuO, Cr- O l e- 0 FeO, V 0 CoO, Nd O and elemental Au. 

1. A GLASS-CERAMIC ARTICLE CONSISTING ESSENTIALLY OF TETRASILICIC FLUORMICA CRYSTALS DISPERSED IN A GLASSY MATRIX, SAID CRYSTALS CONSTITUTING AT LEAST 50% BY VOLUME OF THE ARTICLE AND BEING ESSENTIALLY FREE OF AL+3 AND B+3 AS CRYSTAL CONSTITUENTS, AND SAID ARTICLE BEING FORMED BY THE CRYSTALLIZATION IN SITU OF A GLASS BODY CONSISTING ESSENTIALLY, IN WEIGHT PERCENT ON THE OXIDE BASIS AS CALCULATED FROM THE BATCH, OF ABOUT 55-65% SIO2, 12-20% MGO, 9-13% MGF2, 5-35% (R2O + RO) WHEREIN R2O RANGES FROM 5-25% AND CONSISTS OF AT LEAST ONE OXIDE SELECTED IN THE INDICATED PROPORTION FROM THE GROUP CONSISTING OF 0-20% K2O, 0-23% RB2O AND 0-25% CS2O, AND WHEREIN RO RANGES FROM 0-20% AND CONSISTS OF AT LEAST ONE OXIDE SELECTED FROM THE GROUP CONSISTING OF SRO, BAO AND CDO, 0-10% TOTAL OF OXIDES SELECTED FROM THE GROUP CONSISTING OF AS2O5 AND SB2O5, AND 0-2% TOTAL OF GLASS COLORANTS SELECTED FROM THE GROUP CONSISTING OF K2CR2O7, KMNO4, NIO, CUO, CR2O3, FE2O3, FEO, V2O5, COO, ND2O3 AND ELEMENTAL AU.
 2. A glass-ceramic article consisting essentially of tetrasilicic fluormica crystals dispersed in a glassy matrix, said crystals constituting at least 50% by volume of the article and being essentially free of Al 3 and B 3 as crystal constituents, and said article being formed by crystallization in situ of a glass body consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 55-65% SiO2, 12-20% MgO, 9-13% MgF2, 7-18% K2O and 0.5-8% As2O5.
 3. A glass-ceramic article according to claim 2 which additionally contains from a trace to about 2% of glass colorants selected from the group consisting of K2Cr2O7, KMnO4, NiO, CuO, Cr2O3, Fe2O3, FeO, V2O5, CoO, Nd2O3 and elemental Au. 